Digital breast tomosynthesis (DBT) is an imaging technique that allows a volumetric reconstruction of the whole breast from a finite number of projections obtained by different x-ray source angles. DBT is an important tool used for screening and diagnostic mammography. This technique involves taking a series of x-ray images (projections, also called views) with the x-ray source at different positions while the detector and breast are relatively stationary. The source emits x-rays from a focal spot location in the source. At any point in a DBT scan, a line between the focal spot and the center of the detector defines a projection angle relative to a direction perpendicular to the detector plane. The projection angle should be relatively constant during acquisition of the image in order to map features in the breast to a relatively fixed locations on the detector. In conventional DBT the x-ray source makes an arc, during which a series of images is acquired at different relatively fixed projection angles. Alternately, the x-ray source can move along a linear path as is practiced today for a chest tomography, a related 3D imaging method. In another approach, the x-ray source remains stationary and the detector is moved along a predetermined path. During the motion of the x-ray source, a static or dynamic collimator stationed at the x-ray source exit will direct the x-ray field so as to illuminate only the area of the detector. The acquired data is processed by a computer, where a reconstruction algorithm combines the projections from known projection angles to obtain sectional views of the breast.
Current systems use either a step-and-shoot configuration, where the x-ray source (or detector) is stationary during x-ray exposure, or a continuous motion configuration, where the x-ray source (or detector) is constantly moving but the x-rays are pulsed during the motion. In the former configuration the relatively fixed projection angle is ensured by the stationary source location; in the latter configuration the projection angle is ensured by the short temporal duration of the pulse. The number of x-ray exposure cycles corresponds to the number of stationary positions or to the number of pulses respectively. In one-to-one correspondence with X-ray exposure cycles are the detector frames, each of which includes an integration and read period. The detector integration period temporally overlaps the corresponding x-ray exposure cycles. Subsequent to the x-ray exposure cycle, the detector read periods is when x-ray data is transferred into digital memory. During the time between each cycle, the x-ray intensity is zero so as to allow the system to move to the next angle location. The detector read periods typically occur during this time between cycles. Movement between locations is often at a higher velocity than the velocity while the x-ray is being pulsed. In both these cases the x-ray source is not run at full duty cycle, being off at least long enough to read out the detector and to move system components into the next angle position.
In continuous motion systems with a pulsed x-ray source there is substantial image blurring that occurs because a single detector integration period is acquired while the x-ray source (or detector) is moving during the x-ray exposure. To minimize this blurring, one option is to increase the x-ray source power and pulse with shorter exposure times. Higher power x-ray sources can cost more and weigh more and release more heat into the system. The higher weight leads to additional system cost since larger motors and more rigid gantries are needed.